Recently, due to the spread of communication devices, which are typified by portable telephones, there have been developed a filter element for radio frequency communications having a feature of passing only electric signals in a particular frequency band by a combination of a plurality of resonators using a surface acoustic wave (SAW) or a bulk acoustic wave (BAW) using a piezoelectric material. Previously, a dielectric filter and a SAW filter were mainly used. Recently, attention has been being drawn to a filter configured to use a piezoelectric thin-film resonator, which is an element having good radio frequency characteristics and capable of downsizing and monolithic forming.
The piezoelectric thin-film resonators have an FBAR (Film Bulk Acoustic Resonator) type and an SMR (Solidly Mounted Resonator) type. The former has an upper electrode/piezoelectric film/lower electrode structure on a substrate as a main structural element, and has a cavity below the lower electrode in an area in which the upper electrode and the lower electrode face each other. Here, the cavity is formed by wet etching of a sacrificed layer provided on the substrate surface on which the lower electrode is disposed, or wet or dry etching of the substrate from its back surface. The latter has a structure using, instead of the above cavity, an acoustic reflection film formed by alternatively stacking a film having a high acoustic impedance and a film having a low acoustic impedance to a thickness of λ/4 (λ is the wavelength of the acoustic wave).
When a radio frequency signal is applied between the upper and lower electrodes of the piezoelectric thin-film resonator as an electric signal, an acoustic wave is excited within the piezoelectric film sandwiched between the upper and lower electrodes due to the inverse piezoelectric effect. Also, a deformation caused by the acoustic wave is converted into an electric signal due to the piezoelectric effect. Since the acoustic wave is totally reflected by an interface between the upper electrode film and air and an interface between the lower electrode film and air, the acoustic wave is a longitudinal vibration wave having main displacements in the thickness direction of the piezoelectric thin-film. By utilizing such a resonance phenomenon, a resonator (or a filter formed by connecting a plurality of resonators together) having a desired frequency characteristic is obtained.
For example, in the piezoelectric thin-film resonator of FBAR type, resonance takes place at frequencies (H=nλ/2) at which the total film thickness H of the multilayered structure having the main structural element of the upper electrode film/piezoelectric film/lower electrode film formed above the cavity is integer multiples (n times) of ½ of the wavelength λ of the acoustic wave. Assuming that V is the propagation velocity of the acoustic wave defined by the material of the piezoelectric film, the resonance frequency F is expressed asF=nV/(2H).Thus, the resonance frequency F can be controlled by the total thickness H of the multilayered structure.
As a structure of the filter using such a piezoelectric thin-film resonator, there is a ladder type filter in which resonators are connected in series and in parallel in a ladder form. The ladder type filter is widely used because insertion loss and the out-of-band suppression can be easily handled by simply changing the number of stages of the ladder form and the capacitance ratio of the resonators arranged in series and parallel, and the design procedure is simple. There is a lattice type filter having a similar design method.
The filter structure is composed of resonators of a series arm and a parallel arm having multiple frequencies (the frequency relationship: parallel arm<series arm), and it is required to form the resonators having the multiple resonance frequencies in a common chip. A resonator connected in a parallel arm (hereinafter, parallel resonator) is designed to have a frequency lower than that of a resonator connected in a series arm (hereinafter; series resonator), and a mass addition film is formed on the upper electrode of the parallel resonator, the frequency being controlled by the mass of the mass addition film.
Patent Document 1 discloses a method of varying the thicknesses of the lower electrode, the piezoelectric film and the upper electrode, which are primary structural films of the resonator, and a method of adjustment by adding a mass addition film in order to realize resonators having resonance frequencies on a common substrate. Patent Document 2 discloses a method of adjustment by patterning a mass addition film on an electrode of a resonator.